Method for producing Al-Mg-Si alloy sheet excellent in bake-hardenability and hemmability

ABSTRACT

[PROBLEMS] To provide a method for producing an aluminum alloy sheet excellent in bake-hardenability and hemmability at a low cost by the employment of a very short production process.  
     [MEANS FOR SOLVING PROBLEMS] A method for producing an aluminum alloy sheet, which comprises providing an aluminum alloy melt having a chemical composition, in wt %, that Mg: 0.30 to 1.00%, Si: 0.30 to 1.20%, Fe: 0.05 to 0.50%, Mn: 0.05 to 0.50%, Ti: 0.005 to 0.10%, optionally further one or more of Cu: 0.05 to 0.70% and Zr: 0.05 to 0.40%, and the balance: Al and inevitable impurities, casting the alloy melt into a slab having a thickness of 5 to 15 mm by the twin belt casting method with a cooling speed at ¼ of the thickness of the slab of 40 to 150° C./s, winding up a coil, subjecting the coil to a homogenizing treatment, cooling the resultant coil to a temperature of 250° C. or lower with a cooling speed of 500° C./hr or more, followed by cold rolling, and then subjecting the resulting product to a solution treatment.

TECHNICAL FIELD

The present invention relates to a production method for obtaining anAl—Mg—Si alloy sheet that is abundant in hemmability whilesimultaneously having a high age-hardening ability, by casting a thinslab by continuous casting of an Al—Mg—Si alloy, performing ahomogenization treatment, then cold rolling, and performing a solutiontreatment in a continuous annealing furnace as needed. According to thepresent method, it is possible to produce, at a low cost as compared tothe conventional art, rolled sheets of Al—Mg—Si alloy that are suitablefor forming by bending, press forming and the like of automotive parts,household appliances and the like.

BACKGROUND ART

Al—Mg—Si alloys have the property of increasing in strength when heat isapplied during processes such as coating after forming, so that they arewell-suited for use in automotive panels or the like. Furthermore, theproduction of sheets of the alloys by continuous casting and rolling hasbeen proposed to reduce costs by improved productivity.

For example, Japanese Patent Application, First Publication No.S62-207851 discloses an aluminum alloy sheet for forming and method ofproduction thereof, obtained by continuous casting of an aluminum alloymelt comprising 0.4-2.5% Si, 0.1-1.2% Mg and one or more among 1.5% orless of Cu, 2.5% or less of Zn, 0.3% or less of Cr, 0.6% or less of Mnand 0.3% or less of Zr, to form a 3-15 mm thick slab, cold rolling, thenperforming a solution treatment and quenching, characterized in that themaximum size of intermetallic compounds in the matrix is 5 μm or less.

Japanese Patent Application, First Publication No. H10-110232 disclosesan Al—Mg—Si alloy sheet, obtained by preparing a direct cast rolledsheet of Al alloy comprising 0.2-3.0% Si and 0.2-3.0% Mg, containing oneor more of 0.01-0.5% Mn, 0.01-0.5% Cr, 0.01-0.5% Zr and 0.001-0.5% Ti,and further containing 0-2.5% Cu, 0-0.20% Sn and 0-2.0% Zn, with Febeing limited to 1.0% or less and the remainder consisting of Al andunavoidable impurities, and further cold rolling, characterized in thatthe maximum crystal size in the metallic portion of the sheet is 100 μmor less and the maximum length of continuous Mg₂Si compounds on thesurface layer portion is 50 μm or less.

Additionally, Japanese Patent Application, First Publication No.2001-262264 proposes an Al—Mg—Si alloy sheet excelling in ductility andbendability, the aluminum alloy comprising 0.1-2.0% Si, 0.1-2.0% Mg,0.1-1.5% Fe or one or more further elements chosen from among 2% or lessof Cu, 0.3% or less of Cr, 1.0% or less of Mn, 0.3% or less of Zr, 0.3%or less of V, 0.03% or less of Ti, 1.5% or less of Zn and 0.2% or lessof Ag, wherein the maximum size of intermetallic compounds is 5 μm orless, the maximum aspect ratio is 5 or less and the average crystalgrain size is 30 μm or less.

Patent Document 1: Japanese Patent Application, First Publication No.S62-207851

Patent Document 2: Japanese Patent Application, First Publication No.Hi0-110232

Patent Document 3: Japanese Patent Application, First Publication No.2001-262264

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Alloy sheets that are used as outer panels in automotive body sheets orthe like require exceptional hemmability and bake-hardenability. Forthis reason, Al—Mg—Si alloy sheets that excel in bendability andage-harden when heated have been sought. However, sheets produced bycontinuous casting and rolling have the drawbacks of poor hemmabilityand insufficient bake-hardenability after coating.

The problem to be solved by the present invention is to obtain, at a lowcost, an Al—Mg—Si alloy sheet for forming that suppresses GP zones thatare deposited during natural ageing when left at room temperature,achieves a high level of bake-hardening due to a reinforcement phasebeing quickly deposited upon heating during coating and baking, whilesimultaneously having abundant bendability.

MEANS FOR SOLVING THE PROBLEMS

A thin slab of Al—Mg—Si alloy is continuously cast by a twin-beltcasting machine, the cast thin slab is directly wound, subjected to ahomogenization treatment under appropriate conditions, and cold rolled,then combined with a solution treatment in a continuous annealingfurnace as needed, thereby fragmenting the compounds and raising thehemmability while simultaneously enabling the procedure to beconsiderably shortened. Furthermore, microsegregation is reduced by ahomogenization treatment, and the cooling rate after the homogenizationtreatment is raised, thereby reducing the deposition of Mg₂Si whilecooling, to obtain an aluminum sheet for automotive body sheets withexcellent bake-hardenabiltiy and hemmability after a final anneal.

The present invention which solves the above problem relates to a methodof producing aluminum alloy sheets characterized by winding into thinslabs, subjecting to a homogenization treatment, cold rolling, thensubjecting to a solution treatment. Specifically, as recited in claim 1,it is a method of producing aluminum alloy sheets excelling inbake-hardenability and hemmability, comprising steps of casting, bymeans of a twin-belt casting method, an alloy melt comprising 0.30-1.00wt % of Mg, 0.30-1.20 wt % of Si, 0.05-0.50 wt % of Fe, 0.05-0.50 wt %of Mn and 0.005-0.10 wt % of Ti, optionally further comprising at leastone of 0.05-0.70 wt % of Cu or 0.05-0.40 wt % of Zr, the remainderconsisting of Al and unavoidable impurities, to form a 5-15 mm thickslab at a cooling rate of 40-150° C./s at a quarter-thickness of theslab; winding into a coil; subjecting to a homogenization treatment;cooling to 250° C. or less at a cooling rate of at least 500° C./h; coldrolling; then subjecting to a solution treatment (invention according toclaim 1).

In the above production method, the homogenization treatment preferablyinvolves heating to 520-580° C. at a heating rate of at least 30° C./hin a batch furnace, then holding at that temperature for 2-24 hours(invention according to claim 2).

The solution treatment preferably involves heating to 530-560° C. at aheating rate of at least 10° C./s in a continuous annealing line, andholding for 30 seconds or less (invention according to claim 3).

Furthermore, in the invention according to claim 3 mentioned above, thesolution treatment may be followed by steps of cooling to roomtemperature at a cooling rate of at least 10° C./s, then subjecting to arestoration treatment by holding for 30 seconds or less at 260-300° C.in a continuous annealing furnace, and cooling to room temperature at acooling rate of at least 10° C./s (invention according to claim 4).

Alternatively, in the invention according to claim 3 mentioned above,the solution treatment may be followed by steps of water-cooling to 250°C. or less at a cooling rate of at least 10° C./s, then air-cooling to60-100° C. at a cooling rate of 1-20° C./s, coiling up, and subjectingto a preliminary ageing treatment by cooling to room temperature(invention according to claim 5).

Alternatively, in the invention according to claim 3 mentioned above,the solution treatment may be followed by steps of cooling to roomtemperature at a cooling rate of at least 10° C./s, then subjecting to arestoration treatment by holding for 30 seconds or less at 260-300° C.in a continuous annealing furnace, cooling to 60-100° C. at a coolingrate of at least 1° C./s, coiling up, and subjecting to a preliminaryageing treatment by cooling to room temperature (invention according toclaim 6).

EFFECTS OF THE INVENTION

According to the aluminum alloy sheet production method of the presentinvention, it is possible to obtain an aluminum alloy sheet withexceptional hemmability and bake-hardenability. Additionally, thisproduction method is capable of obtaining an aluminum alloy sheet in anextremely short procedure and at low cost.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method of producing a rolled sheet ofAl—Mg—Si alloy, characterized by casting a thin slab by a twin-beltcasting method, winding the slab directly onto a coil, subjecting to ahomogenization treatment, then cold rolling, and further subjecting to asolution treatment.

In the present invention, an alloy melt consisting of the aforementionedcomposition is cast into a slab 5-15 mm thick at a cooling rate of40-150° C./s at a quarter thickness of the slab, using a twin-beltcasting method, and after winding into a coil, it is subjected to ahomogenization treatment and cooled to 250° C. or less at a cooling rateof at least 500° C./s, then cold rolled, and subsequently subjected to asolution treatment.

The twin-belt casting method is a method of casting thin slabs bypouring a melt between water-cooled rotating belts that oppose eachother from above and below, so as to harden the melt by cooling throughthe belt surfaces. In the present invention, slabs that are 5-15 mmthick are cast by the twin-belt casting method. If the slab thicknessexceeds 15 mm, it becomes difficult to wind the thin slabs into coils,and if the slab thickness is less than 5 mm, there is a loss inproductivity and it becomes difficult to cast the thin slabs.

By casting a slab 5-15 mm thick using the twin-belt casting method, itis possible to make the cooling rate 40-150° C./s at a quarter thicknessof the slab. The cooling rate is computed by measuring the DAS (DendriteArm Spacing) by a line intersection method from observations of themicrostructure in the slab at quarter thickness. When the cooling rateis less than 40° C./s, the cast structure formed in the central portionof the slab during hardening becomes coarse, thus reducing thehemmability, while if the cooling rate exceeds 150° C./s, Al—Fe—Sicrystals and Al—(Fe.Mn)—Si crystals become 1 μm or less and the size ofrecrystallized grains becomes coarse at 30 μm or more.

After winding a thin slab, this coil is subjected to a homogenizationtreatment under appropriate conditions to fragment the Al—Fe—Si crystalsand Al—(Fe.Mn)—Si crystals that have an adverse effect on hemmability,thus improving the hemmability. Furthermore, it is possible to obtainthin slabs in a state where relatively small Mg₂Si crystals that residein the cast structure are completely dissolved into the matrix, thusraising the effectiveness of the solid solution treatment after the coldrolling process.

The reason that the cooling after the homogenization treatment isperformed at a rate of at least 500° C./s and to 250° C. or less is inorder to suppress the deposition of relatively coarse Mg₂Si as much aspossible, and to dissolve the Mg and Si into the matrix in anoversaturated state.

After winding the thin slab, the coil is inserted into a batch furnace,and heated at a rate of at least 30° C./h to 520-580° C., at whichtemperature it is held for 2-24 hours to perform a homogenizationtreatment, after which the coil may be extracted from the batch furnaceand forcibly air-cooled to room temperature at a cooling rate of atleast 500° C./h. This cooling can be performed, for example, by a fanwhile unwinding the coil.

The reason the heating rate to the homogenization temperature is limitedto at least 30° C./h for the homogenization treatment following windingof the thin slab is that if the heating rate is less than 30° C./h, atleast 16 hours will be required to reach the predeterminedhomogenization temperature, thus raising costs.

The reason the homogenization temperature is within the range of520-580° C. is that if the temperature is less than 520° C., thefragmentation of Al—Fe—Si crystals and Al—(Fe.Mn)—Si crystals isinadequate, and not enough to dissolve the Mg₂Si that crystallizedduring casting into the matrix, and if the temperature exceeds 580° C.,the metals with low melting points will melt and cause burning.

Additionally, the reason that the homogenization treatment time is setto within the range of 2-24 hours is because if the treatment time isless than 2 hours, the fragmentation of Al—Fe—Si crystals andAl—(Fe.Mn)—Si crystals is inadequate, and not enough to dissolve theMg₂Si that crystallized during casting into the matrix, and if thetreatment time exceeds 24 hours, the Mg₂Si that crystallized duringcasting is well-dissolved into the matrix, and the Mg and Si becomesaturated, resulting in cost increases.

The invention is characterized by further cold rolling this coil andperforming a solution treatment. This solution treatment is preferablyperformed in a normal continuous annealing line (CAL).

A continuous annealing line (CAL) is an installation for performingcontinuous solution treatments and the like of coils, characterized bycomprising inductive heating devices for performing heat treatments,water tanks for water-cooling, air nozzles for air-cooling, and thelike.

As for the solution treatment, it should preferably be performed byheating at a rate of at least 10° C./s to 530-560° C. by means of acontinuous annealing line, and holding for 30 seconds or less.

The reason the heating rate to the solution treatment temperature islimited to at least 10° C./s in the solution treatment is that if theheating rate is less than 10° C./s, the coil advancing speed becomes tooslow, as a result of which the processing time becomes long and the costmounts.

The reason the solution treatment temperature is set to be within therange of 530-560° C. is that if the temperature is less than 530° C., itis not sufficient to cause Mg₂Si that crystallized while casting orprecipitated while being cooled after homogenization to be dissolvedinto the matrix, and if the temperature exceeds 560° C., the metals withlow melting points will melt and cause burning.

Additionally, the reason the solution treatment time is restricted to bewithin 30 seconds is that in the case of treatment times exceeding 30seconds, Mg₂Si that crystallized while casting or precipitated whilebeing cooled after homogenization is well-dissolved into the matrix, andthe Mg and Si become saturated, thereby slowing the coil advancementspeed, as a result of which the processing time is increased and thecosts mount.

The invention is characterized by cooling to room temperature at a rateof at least 10° C./s after the solution treatment. The reason thecooling rate after the solution treatment is at least 10° C./s is thatif the cooling rate is less than 10° C./s, Si is deposited in thecrystal grain boundary during the cooling step, thus reducing thehemmability.

After performing the aforementioned homogenization treatment on the thinslab, it is further cold rolled, subjected to a solution treatment andcooled to room temperature at a rate of at least 10° C./s, and after thecoil is left at room temperature, it may be held for 30 seconds or lessat 260-300° C. in a continuous annealing line, then cooled to roomtemperature at 10° C./s.

This solution treatment and restoration treatment are preferablyperformed in a normal continuous annealing line. A continuous annealingline (CAL) is an installation for performing continuous solutiontreatments and the like of coils, characterized by comprising inductiveheating devices for performing heat treatments, water tanks forwater-cooling, air nozzles for air-cooling, and the like. Due to therestoration treatment, it is possible to re-dissolve GP zones thatappear due to natural ageing when left at room temperature after asolution treatment, thus enabling adequate strength to be obtained afterheating for coating and baking.

Additionally, in order to obtain adequate strength after heating forcoating and baking, it is left at room temperature after the solutiontreatment and subjected to a restoration treatment at 260-300° C. If therestoration treatment temperature is less than 260° C., adequatebake-hardenability cannot be obtained, and if it exceeds 300° C., thehemmability is reduced.

The reason the time over which the restoration treatment temperature isheld is restricted to within 30 seconds is that if the treatment timeexceeds 30 seconds, it is not possible to adequately re-dissolve the GPzones that appear due to natural ageing when left at room temperatureafter the solution treatment, in addition to which the coil advancementspeed is too slow, as a result of which the treatment time is long andthe costs mount.

After performing the aforementioned homogenization treatment on the thinslab, it can be further cold rolled, subjected to a heat solutiontreatment in a continuous annealing line, water-cooled to 250° C. orless at a cooling rate (first cooling rate) of at least 10° C./s, thenair-cooled to 60-100° C. at a cooling rate (second cooling rate) of1-20° C./s, coiled up and cooled to room temperature.

This heat solution treatment and subsequent cooling are preferablyperformed in a normal continuous annealing line (CAL). During this heatsolution treatment and subsequent cooling, a heat treatment (preliminaryageing) can be performed to evenly generate nuclei for β″ deposition inthe matrix, to obtain adequate strength after heating for coating andbaking.

After subjecting the thin slab to a homogenization treatment and furthercold rolling, it may be subjected to a solution treatment by heating to530-560° C. at a rate of at least 10° C./s, then holding for 30 secondsor less, then cooled to room temperature at a rate of at least 10° C./s,thereafter subjected to a restoration treatment by holding within arange of 260-300° C. for 30 seconds, then cooled to 60-100° C. at acooling rate of at least 1° C./s, coiled up and subjected to apreliminary ageing treatment by cooling to room temperature.

This solution treatment and subsequent cooling, and restorationtreatment and subsequent cooling are preferably performed in a normalcontinuous annealing line (CAL). With this production method, not onlyis it possible to re-dissolve GP zones that appear due to natural ageingwhen left at room temperature after the solution treatment, but it isalso possible to perform a heat treatment (preliminary ageing) togenerate nuclei for β″ deposition, thus further improving the resistanceafter coating and baking.

Next, the significance of the alloy ingredients of the present inventionand the reasons for their limitations shall be explained. The essentialelement Mg is dissolved in the matrix after the heat solution treatment,and is deposited as a reinforcing phase together with Si upon heatingfor coating and baking, thereby improving the strength. The reason theMg content is limited to 0.30-1.00 wt % is that the effect is small ifless than 0.30 wt %, and if more than 1.00 wt %, the hemmability afterthe solution treatment is reduced. A more preferable range for the Mgcontent is 0.30-0.70 wt %.

The essential element Si is deposited together with Mg as anintermediary phase of Mg₂Si known as β″ or an analogous reinforcingphase upon being heated for coating and baking, thereby increasing thestrength. The reason the Si content is limited to 0.30-1.20 wt % is thatif less than 0.30 wt %, its effects are minimal, and if more than 1.20wt %, the hemmability is reduced after the heat solution treatment. Amore preferable range of Si content is 0.60-1.20 wt %.

The essential element Fe, when coexisting with Si and Mn, generates manyAl—Fe—Si crystals and Al—(Fe.Mn)—Si crystals of a size of 5 μm or lessupon casting, so that re-crystallized nuclei are increased, as a resultof which the recrystallized grains are refined and sheets of exceptionalformability are obtained. If the Fe content is less than 0.05 wt %, theeffects are not very remarkable. If it exceeds 0.50 wt %, coarseAl—Fe—Si crystals and Al—(Fe.Mn)—Si crystals are formed upon casting,thus not only reducing the hemmability but also reducing the amount ofSi dissolved in the thin slabs, as a result of which thebake-hardenability of the final sheets is reduced. Therefore, thepreferable range of Fe content is 0.05-0.50 wt %. A more preferablerange of Fe content is 0.05-0.30 wt %.

The essential element Mn is added as an element to refine there-crystallized grains. By keeping the size of the re-crystallizedgrains relatively small at 10-25 μm, it is possible to form sheets withexceptional formability. If the Mn content is less than 0.05 wt %, theeffect is not adequate, and if it exceeds 0.50 wt %, coarse Al—Fe—Sicrystals and Al—(Fe.Mn)—Si crystals are formed upon casting, thus notonly reducing the hemmability but also reducing the amount of Sidissolved in the thin slabs, as a result of which the bake-hardenabilityof the final sheets is reduced. Therefore, the preferable range of Mncontent is 0.05-0.50 wt %. A more preferable range of Mn content is0.05-0.30 wt %.

The essential element Ti will not inhibit the effects of the presentinvention if it is contained at 0.10 wt % or less, and it can functionas a crystal grain refiner for the thin slabs, so as to reliably preventcasting defects of the slabs such as cracks or the like. If the Ticontent is less than 0.005 wt %, the effects are not adequate, and ifthe Ti content exceeds 0.10 wt %, coarse intermetallic compounds such asTiAl₃ and the like are formed during casting, thus greatly reducing thehemmability. Therefore, the preferable range of Ti content is 0.005-0.10wt %. A more preferable range for the Ti content is 0.005-0.05 wt %.

The optional element Cu is an element that promotes age-hardening andraises the bake-hardenability. If the Cu content is less than 0.05 wt %,the effect is small, and if it exceeds 0.70 wt %, the yield strength ofthe sheets becomes high after a preliminary ageing treatment, and notonly does the hemmability decrease, but the reduction in corrosionresistance is also marked. Therefore, the Cu content is preferablywithin a range of 0.05-0.70 wt %. The Cu content is more preferably0.10-0.60 wt %.

The optional element Zr is added as an element for refining there-crystallized grains. If the Zr content is less than 0.05 wt %, theeffect is not adequate, and if it exceeds 0.40 wt %, coarse Al—Zrcrystals are created during slab casting, thus reducing the hemmability.Therefore, the Zr content is preferably within a range of 0.05-0.40 wt%. The Zr content is more preferably within a range of 0.05-0.30 wt %.

As explained above, the present invention allows an Al—Mg—Si alloy sheetfor use in automotive body sheets having exceptional bake-hardenablitiyand hemmability after a final anneal to be produced at low cost. While arestoration treatment or high-temperature winding is required tosuppress natural ageing as with conventional methods, the steps such asfacing, hot rolling and the like that precede these steps can be largelysimplified, thus greatly reducing the total production cost.

Herebelow, the best modes of the present invention shall be describedusing examples.

EXAMPLE 1

In the below-given examples, the samples after cold rolling are notcoils but all cut sheets. Therefore, in order to simulate the step ofcontinuous annealing of a coil in a continuous annealing line (CAL), asolution treatment of the samples in a salt bath and a cold water quenchor 85° C. water quench were employed.

After degassing melts having the compositions shown in Table 1, theywere cast into slabs 7 mm thick by means of a twin-belt casting method.The DAS (Dendrite Arm Spacing) was measured by an intersection methodfrom observation of the microstructures at a quarter-thickness of theslab, and the cooling rate 75° C./s was computed. A predeterminedhomogenization treatment was performed on the slabs which were thencooled to room temperature at a predetermined cooling rate, and coldrolled to form sheets of 1 mm thickness. Next, solution treatments wereperformed on these cold rolled sheets in a salt bath, and they wereeither 1) quenched in 85° C. water and immediately inserted into anannealer with a predetermined atmospheric temperature to perform a heattreatment under predetermined conditions, or 2) quenched in cold water,left at room temperature for 24 hours, then subjected to a heattreatment under predetermined conditions. Furthermore, in order tosimulate automobile coating steps, they were held for one week at roomtemperature after the heat treatment, and measured for 0.2% yieldstrength, further baked at 180° C. for 30 minutes, and again measuredfor 0.2% yield strength.

The difference in yield strength before and after the baking treatmentwas taken as the bake-hardenability, and those exceeding 80 MPa werejudged to have excellent bake-hardenability. In order to simulatehemmability, the sheets prior to baking were preliminarily warped by 5%,then bent into a U shape using a jig having a radius r=0.5 mm, then 1 mmthick spacers were inserted and they were bent 180°. Those which did notcrack were ranked ◯ and those which cracked were ranked X. The detailedsheet production steps and evaluation results are shown in Table 2-6.

[Table 1] TABLE 1 Alloy Composition (wt %) Alloy No. Mg Si Fe Mn Cu ArTi A 0.5 0.7 0.2 0.2 — — 0.02 B 0.5 0.8 0.2 0.2 — — 0.02 C 0.6 0.8 0.20.2 — — 0.02 D 0.5 1 0.2 0.2 0.5 — 0.02 E 0.5 0.8 0.2 0.2 — 0.15 0.02 F0.4 1.2 0.2 0.2 0.1 — 0.02

Table 2 shows the results for cases in which the homogenizationconditions and cooling rate after the homogenization treatment werechanged. After the homogenization treatment, the slabs were cold rolledto a thickness of 1 mm, these cold rolled sheets were subjected to asolution treatment by holding for 15 seconds at a predeterminedtemperature by means of a salt bath, then quenched with 85° C. water,and immediately inserted into an annealer with an atmospherictemperature of 85° C. to perform a preliminary ageing of 8 hours. Thosefalling within the scope of conditions of the present invention (1-7)had exceptional bake-hardenability and hemmability. Those that did notundergo a homogenization treatment (8, 10) had poor bake-hardenabilityand hemmability. Additionally, those which had a slow cooling rate afterthe homogenization treatment had poor bake-hardenability (9).

[Table 2] TABLE 2 Cooling Rate after Homogenization andBake-Hardenability/Hemmability Homogenization Treatment Cast Type/Heating Holding Holding Cooling Alloy Slab Thick. Rate Temp Time Rate IDNo. (mm) (° C./h) (° C.) (h) (° C./h) Present 1 A twin-belt/7 30 560 51500 Invention 2 B twin-belt/7 50 560 6 1700 3 B twin-belt/7 50 550 5500 4 C twin-belt/7 30 530 10 1000 5 D twin-belt/7 40 530 10 1000 6 Etwin-belt/7 40 530 10 1000 7 F twin-belt/7 50 550 6 1000 Comp. 8 Atwin-belt/7 None Example 9 B twin-belt/7 50 560 6 250 10 B twin-belt/7None Cold Roll Sol. Yield Str. Bake- Sheet Treat. Prelim. before/afterHard. ID Thick. Temp. Ageing Baking (Mpa) (MPa) Hem. Present 1 1 mm 550°C. 85° C. × 8 h 100/192 92 ◯ Invention 2 1 mm 550° C. 85° C. × 8 h110/210 100 ◯ 3 1 mm 530° C. 85° C. × 8 h  95/175 80 ◯ 4 1 mm 540° C.85° C. × 8 h 107/209 102 ◯ 5 1 mm 550° C. 85° C. × 8 h 122/221 99 ◯ 6 1mm 550° C. 85° C. × 8 h 115/213 98 ◯ 7 1 mm 550° C. 85° C. × 8 h 117/20891 ◯ Comp. 8 1 mm 550° C. 85° C. × 8 h 110/158 48 X Example 9 1 mm 550°C. 85° C. × 8 h  90/145 55 ◯ 10 1 mm 550° C. 85° C. × 8 h  92/160 68 X

Table 3 shows the results when the temperatures/times of thehomogenization treatment are changed. After the homogenizationtreatment, the slabs were cold rolled to a thickness of 1 mm, these coldrolled sheets were subjected to a solution treatment by holding for 15seconds at a predetermined temperature by means of a salt bath, thenquenched in 85° C. water and immediately entered into an annealer withan atmospheric temperature of 85° C. to perform a preliminary ageing of8 hours. Those falling within the scope of conditions of the presentinvention (11-14) had exceptional bake-hardenability and hemmability.Those that had a low homogenization temperature (15) or had a shortholding time (16) had poor bake-hardenability and hemmability.

[Table 3] TABLE 3 Homogenization Temperature/Time andBake-Hardenabilit/Hemmability Homogenization Treatment Cast Type/Heating Holding Holding Cooling Alloy Slab Thick. Rate Temp Time Rate IDNo. (mm) (° C./h) (° C.) (h) (° C./h) Present 11 B twin-belt/7 30 560 51500 Invention 12 B twin-belt/7 50 560 6 1500 13 C twin-belt/7 50 550 51500 14 C twin-belt/7 30 530 10 1500 Comp. 15 B twin-belt/7 50 500 61500 Example 16 B twin-belt/7 50 560 1 1500 Cold Roll Sol. Yield Str.Bake- Sheet Treat. Prelim. before/after Hard. ID Thick. Temp. AgeingBaking (Mpa) (MPa) Hem. Present 11 1 mm 550° C. 85° C. × 8 h 110/210 100◯ Invention 12 1 mm 550° C. 85° C. × 8 h 111/213 103 ◯ 13 1 mm 530° C.85° C. × 8 h 107/209 102 ◯ 14 1 mm 540° C. 85° C. × 8 h 112/215 103 ◯Comp. 15 1 mm 550° C. 85° C. × 8 h  95/165 70 X Example 16 1 mm 550° C.85° C. × 8 h 100/175 75 X

Table 4 shows the results when the homogenization conditions andrestoration conditions were changed. After the homogenization treatment,the slabs were cold rolled to a thickness of 1 mm, these cold rolledsheets are subjected to a solution treatment by holding for 15 secondsat a predetermined temperature by means of a salt bath, then quenched incold water, and after leaving at room temperature for 24 hours,subjected to a restoration treatment by holding for 15 seconds at apredetermined temperature. Those falling within the scope of conditionsof the present invention (17-20) had exceptional bake-hardenability andhemmability. Those that had a low restoration temperature (reheatingtemperature) (21) had poor bake-hardenability. Those whose restorationtemperature (reheating temperature) was too high (22) had poorhemmability. Furthermore, even when the restoration conditions werewithin the scope of the present invention, those in which thehomogenization temperature was low (23) or the holding time was short(24) had poor hemmability. Those in which the cooling rate after thehomogenization treatment was slow (25) had poor bake-hardenability.

[Table 4] TABLE 4 Homogenization Method/Reheat Temperature andBake-Hardenability/Hemmability Homogenization Treatment Cast Type/Heating Holding Holding Cooling Alloy Slab Thick. Rate Temp Time Rate IDNo. (mm) (° C./h) (° C.) (h) (° C./h) Present 17 B twin-belt/7 30 560 51500 Invention 18 B twin-belt/7 50 560 6 2000 19 C twin-belt/7 50 550 51000 20 C twin-belt/7 30 530 10 2500 Comp. 21 B twin-belt/7 50 560 61500 Example 22 B twin-belt/7 50 560 6 1500 23 B twin-belt/7 50 500 6500 24 B twin-belt/7 50 560 1 1000 25 B twin-belt/7 50 560 6 200 ColdRoll Sol. Yield Str. Bake- Sheet Treat. Prelim. before/after Hard. IDThick. Temp. Ageing Baking (Mpa) (MPa) Hem. Present 17 1 mm 550° C. 270110/210 100 ◯ Invention 18 1 mm 550° C. 270 111/213 103 ◯ 19 1 mm 530°C. 290 107/209 102 ◯ 20 1 mm 540° C. 290 112/215 103 ◯ Comp. 21 1 mm550° C. 240  95/170 75 ◯ Example 22 1 mm 550° C. 310 127/229 102 X 23 1mm 550° C. 290  97/197 100 X 24 1 mm 550° C. 280  90/160 70 X 25 1 mm550° C. 290  95/145 50 ◯

Table 5 shows the results when the homogenization conditions and coolingpattern after the solution treatment were changed. The cooling rateafter the solution treatment was divided into two stages, with thecooling rate from the solution temperature to an intermediatetemperature being defined as the first cooling rate and the cooling ratefrom the intermediate temperature to the coil-up temperature beingdefined as the second cooling rate. After the homogenization treatment,the slabs were cold rolled to a thickness of 1 mm, and these cold rolledsheets were subjected to a solution treatment by holding for 15 secondsat a predetermined temperature by means of a salt bath, after which theywere cooled to the intermediate temperature at the first cooling rate,then cooled to the coil-up temperature at the second cooling rate, andthereafter cooled to room temperature at 5° C./h.

Those falling within the scope of the present invention (26-28) hadexceptional bake-hardenability and hemmability. Those in which the firstcooling rate after the solution treatment was slow (29), those in whichthe second cooling rate was slow (31) or those in which the intermediatetemperature was too high (30) had poor hemmability. Those in which thecoil-up temperature was too low (32) had poor bake-hardenability.Conversely, those in which the coil-up temperature was too high (33) hadpoor hemmability. Furthermore, those in which the homogenizationtreatment temperature was too low (34) or the holding time was too short(35) had poor hemmability. Those in which the cooling rate after thehomogenization treatment was too slow (36) had poor bake-hardenability.

[Table 5] TABLE 5 Homogenization Method/Coil-up Temperature andBake-Hardenability/Hemmability Homogenization Treatment Cast Type/Heating Holding Holding Cooling Alloy Slab Thick. Rate Temp Time Rate IDNo. (mm) (° C./h) (° C.) (h) (° C./h) Present 26 B twin-belt/7 30 560 51500 Invention 27 B twin-belt/7 50 560 6 2000 28 B twin-belt/7 50 550 51000 Comp. 29 B twin-belt/7 50 560 6 1500 Example 30 B twin-belt/7 50560 6 1500 31 B twin-belt/7 50 560 6 1500 32 B twin-belt/7 50 560 6 150033 B twin-belt/7 50 560 6 2000 34 B twin-belt/7 50 500 6 1000 35 Btwin-belt/7 50 560 1 1000 36 B twin-belt/7 50 560 6 200 Cold Sol. FirstSec. Coil Roll Treat. Cool Int. Cool Up YS b/a Bake- Sheet Tem. TempTemp Temp Temp Bak. Hard. ID Thick. (° C.) (° C.) (° C.) (° C.) (° C.)(Mpa) (MPa) Hem Present 26 1 mm 550 100 200 20 85 110/210 101 ◯Invention 27 1 mm 550 100 200 20 70 105/207 102 ◯ 28 1 mm 530 100 200 2090 101/211 100 ◯ Comp. 29 1 mm 550 5 200 20 80 106/201 95 X Example 30 1mm 550 100 300 20 80 101/197 96 X 31 1 mm 550 100 250 1 80 102/198 96 X32 1 mm 550 100 200 20 50 112/165 53 ◯ 33 1 mm 550 100 200 15 110130/240 110 X 34 1 mm 550 100 200 20 85  97/197 100 X 35 1 mm 550 100200 20 85 104/194 90 X 36 1 mm 550 100 200 20 80  89/134 45 ◯

Table 6 shows the results when the restoration treatment temperature(reheating temperature) after the solution treatment and coil-uptemperature were changed. After the homogenization treatment, the slabswere cold rolled to a thickness of 1 mm, these cold rolled sheets aresubjected to a solution treatment by holding for 15 seconds at apredetermined temperature by means of a salt bath, then quenched in coldwater, and after leaving at room temperature for 24 hours, held for 15seconds at a predetermined temperature (preheating temperature) andcooled to a predetermined coil-up temperature at 10° C./s, then furthercooled to room temperature at 10° C./h. Those falling within the scopeof conditions of the present invention (37-40) had exceptionalbake-hardenability and hemmability. Those in which the restorationtreatment temperature (reheating temperature) was too high (41) had poorhemmability. Those in which the restoration treatment temperature(reheating temperature) was too low (42) had reduced bake-hardenability.Those in which the coil-up temperature was too low (43) had poorbake-hardenability. Those in which the coil-up temperature was too high(44) had poor hemmability.

[Table 6] TABLE 6 Reheat Temperature/Coil-up Temperature andBake-Hardenability/Hemmability Sol. Reheat Coil Up Yield Str. Bake-Alloy Treat. Temp Temp before/after Hard. ID No. Tem. (° C.) (° C.) (°C.) Baking (Mpa) (MPa) Hem. Present 37 B 550 270 85 121/231 110 ◯Invention 38 B 550 270 90 125/237 114 ◯ 39 B 530 290 70 117/228 111 ◯ 40B 540 290 80 119/231 112 ◯ Comp. 41 B 550 320 85 124/234 110 X Example42 B 550 250 80 111/198 87 ◯ 43 B 550 260 40 110/185 75 ◯ 44 B 550 290120 131/249 118 XHomogenization: 550° C. × 6 hCooling Rate after Homogenization: 1000° C./h

INDUSTRIAL APPLICABILITY

According to the present invention, rolled sheets of Al—Mg—Si alloy thatare suitable for forming by bending, press forming and the like ofautomotive parts, household appliances and the like can be produced at alow cost relative to the conventional art.

1. A method of producing aluminum alloy sheets excelling inbake-hardenability and hemmability, comprising steps of casting, bymeans of a twin-belt casting method, an alloy melt comprising 0.30-1.00wt % of Mg, 0.30-1.20 wt % of Si, 0.05-0.50 wt % of Fe, 0.05-0.50 wt %of Mn and 0.005-0.10 wt % of Ti, optionally further comprising at leastone of 0.05-0.70 wt % of Cu or 0.05-0.40 wt % of Zr, the remainderconsisting of Al and unavoidable impurities, to form a 5-15 mm thickslab at a cooling rate of 40-150° C./s at a quarter-thickness of theslab; winding into a coil; subjecting to a homogenization treatment byinserting the coil into a batch furnace, heating to 520-580° C. at aheating rate of at least 30° C./h, then holding at that temperature for2-24 hours; cooling to 250° C. or less at a cooling rate of at least500° C./h; cold rolling; then subjecting to a solution treatment byheating to 530-560° C. at a heating rate of at least 10° C./s in acontinuous annealing line, and holding for 30 seconds or less. 2.(canceled)
 3. (canceled)
 4. A method in accordance with claim 1,comprising steps, after said solution treatment, of cooling to roomtemperature at a cooling rate of at least 10° C./s, then subjecting to arestoration treatment by holding for 30 seconds or less at 260-300° C.in a continuous annealing furnace, and cooling to room temperature at acooling rate of at least 10° C./s.
 5. A method in accordance with claim1, comprising steps, after said solution treatment, of water-cooling to250° C. or less at a cooling rate of at least 10° C./s, then air-coolingto 60-100° C. at a cooling rate of 1-20° C./s, coiling up, andsubjecting to a preliminary ageing treatment by cooling to roomtemperature.
 6. A method in accordance with claim 1, comprising steps,after said solution treatment, of cooling to room temperature at acooling rate of at least 10° C./s, then subjecting to a restorationtreatment by holding for 30 seconds or less at 260-300° C. in acontinuous annealing furnace, cooling to 60-100° C. at a cooling rate ofat least 1° C./s, coiling up, and subjecting a preliminary ageingtreatment by cooling to room temperature.
 7. A method in accordance withclaim 1 further comprising the step of, after said homogenizationtreatment, removing the coil from the batch furnace and forcibly coolingwhile unwinding the coil.